HD Radio™ digital broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM HD Radio signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band-on-channel (IBOC) BD Radio systems require no new spectral allocations because each HD Radio signal is simultaneously transmitted within the same spectral mask of an existing AM or FM channel allocation. IBOC HD Radio broadcasting promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. An HD Radio digital broadcasting system is described in U.S. Pat. No. 6,549,544, which is hereby incorporated by reference. The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an IBOC standard, designated NRSC-5C, the disclosure of which is incorporated herein by reference, which sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. A copy of the standard can be obtained from the NRSC at http://www.nrscstandards.org.
HD Radio broadcasting systems use a set of orthogonal frequency division multiplexed (OFDM) subcarriers to transmit a digital signal. OFDM modulation is a well-known technique that modulates a vector of information symbols in parallel over multiple subcarriers modulated at different equally spaced frequencies, which are orthogonal to each other. This ensures that different subcarriers do not interfere with each other under normal channel conditions. OFDM modulation has proven to be an effective means for transmission over channels that experience various types of multipath and linear distortion.
A well-known drawback of OFDM is its relatively high PAR. For a large number of subcarriers, each complex dimension (in-phase and quadrature) of the OFDM signal approaches a Gaussian distribution. This results in a signal magnitude (square root of power) Probability Density Function (PDF) that approaches a Rayleigh distribution. Although the Rayleigh distribution has theoretically infinite peaks, the OFDM peak is limited by the number of parallel subcarriers (e.g., 100, or 20 dB), or more practically the typical peak can be limited to about 12 dB since there is little distortion in clipping the improbable tails (e.g., above 12 dB PAR) of the Rayleigh PDF. High Power Amplifier (HPA) power efficiency is affected since a large power backoff is required in operation to minimize peak distortion. This peak distortion not only distorts (i.e., adds noise to) the subcarrier modulation, but unwanted out-of-band emission occurs due to intermodulation distortion. This leakage, being highest immediately outside the intended spectral occupancy, can be particularly difficult to suppress with filters after the HPA output.
Several different types of PAR reduction techniques have been proposed. One peak-to-average power ratio reduction technique is described in U.S. Pat. No. 6,128,350, issued Oct. 3, 2000, and titled “Method And Apparatus For Reducing Peak-To-Average Power Ratio In Digital Broadcasting Systems”. Another peak to average power ratio reduction technique is described in U.S. Pat. No. 7,542,517, issued Jun. 2, 2009, and titled “Peak-To-Average Power Reduction For FM OFDM Transmission”. Another peak-to-average power ratio reduction technique is described in U.S. Pat. No. 8,798,196, issued Aug. 5, 2014, and titled “Peak-To-Average Power Ratio Reduction For Hybrid FM HD Radio Transmission”. U.S. Pat. Nos. 6,128,350; 7,542,517; and 8,798,196 are hereby incorporated by reference. However, these PAR reduction techniques do not address PAR reduction when the signal of interest includes QAM modulation.